Low temperature, nanostructured ceramic coatings

ABSTRACT

A substrate subject to degradation at temperatures above 100° C. is coated with a nanostructured ceramic coating having a thickness in excess of 100 nm, formed on a surface of the substrate, wherein a process temperature for deposition of the nanostructured coating does not exceed 90° C. The coating may be photocatalytic, photovoltaic, or piezoelectric. The coating, when moistened and exposed to ultraviolet light or sunlight, advantageously generates free radicals, which may be biocidal, deodorizing, or assist in degradation of surface deposits on the substrate after use. The substrate may be biological or organic, and may have a metallic or conductive intermediate layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Division of U.S. patent application Ser. No. 14/734,520, filed Jun. 9, 2015, now U.S. Pat. No. 10,828,400, issued Nov. 10, 2020, which is a non-provisional of, and claims priority under 35 U.S.C. § 119(e) to, U.S. Provisional Patent Application No. 62/010,003, filed Jun. 10, 2014, the entirety of which is expressly incorporated herein by reference.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with government support under NNX09AT30G awarded by NASA. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the field of nano-ceramic coatings, and more particularly a process for depositing nanostructured ceramic coatings (e.g., low dimensional nanostructures such as nanoparticles, nanorods, nanoblades, etc.) on plastic substrates at low temperatures, and nano-ceramic coated plastic products and other substrates.

BACKGROUND OF THE INVENTION

Modifying material surfaces to enhance wear and corrosion resistance is a common practice for both military and commercial applications. Electrodeposited hard chrome is one of the most widely used protective coatings. Ceramic coatings, both single phase and composite types, are also common, and they are often applied using plasma spray. In this process, the coating material (usually in the form of a powder) is injected into a hot plasma stream, where it is heated and accelerated toward the substrate surface. After impacting the surface, the ceramic rapidly cools thus forming a coating layer.

Ceramic coatings have serious deficiencies that can limit their use. Plasma-sprayed ceramic coatings are somewhat less expensive than chrome (when clean-up costs are included), but are generally brittle and have limited success adhering to substrates, which is also a problem for hard chrome. The need for better coating materials has been recognized and considerable effort has recently gone into finding replacements.

Nanostructured materials are characterized by an ultra-fine microstructure with some physical aspect less than 100 nanometers in size. This feature can be grain size, particle or fiber/rod diameter, or layer thickness. There are two reasons why reducing the scale of a material's microstructure can significantly alter its properties. First, as grain size gets smaller, the proportion of atoms at grain boundaries or on surfaces increases rapidly. In a polycrystalline material with a grain size of 10 nm, as many as 50% of its atoms are at grain boundaries, resulting in a material with properties different significantly from the normal properties of the corresponding bulk (non-nanostructured) material. Second, many physical phenomena (such as dislocation generation, ferromagnetism, or quantum confinement effects) are governed by a characteristic length. As the physical scale of the material falls below this length, properties change radically.

Until recently, changes in deformation behavior and modes of failure as a result of nanostructuring of materials have not been well understood due to the inability to consistently fabricate high quality materials. This situation is changing rapidly, with considerable progress now being made in the fabrication of nanomaterials, as well as and the understanding of the interrelations between nanoscale processing, structure, and macroscale properties.

Plasma spray, one of the common processes used to fabricate ceramic coatings, is very simple in concept, but very complex in practice. An inert gas is passed through a region of electrical discharge, where it is heated to very high temperature (typically 10,000 to 20,000 K). The rapidly expanding plasma is forced out through a nozzle at velocities between 1,200 and 1,500 m/sec and directed toward a substrate. Particles are injected into the plasma, where they are heated and accelerated. Although the plasma and particle temperatures are high, and surface temperatures during the process are high, deep substrate heating is minimal. The complexity arises from the large number of parameters that must be selected and which can affect the structure and properties of the coating. The temperature and velocity of the plasma depend on the power applied to the gun, and the type and flow rate of the gas used. Usually, two gases are used, an inert gas such as helium or argon, and a secondary gas, such as hydrogen. Other factors include the morphology of the powder particles, distance from the gun to the substrate, position and orientation of the powder injection ports, and surface preparation of the substrate. Taken all together, these parameters determine the thermal history of the injected particles, velocity of impact, and flow and solidification characteristics after impact, thus dictating the resultant microstructure.

As compared to traditional plasma spray processes, plasma spraying of nanostructured materials introduces a number of complications. The first is that nanoparticles cannot be sprayed by particle injection into the plasma. Very small particles lack the momentum necessary to penetrate into the plasma, or to impact the surface while the plasma sweeps to the side near the substrate. To be sprayed, the particles must be formed into agglomerates approximately 30-100 microns in diameter. For an Al₂O₃—TiO₂ nanocomposite, this is usually accomplished by dispersing alumina and titania nanoparticles in a fluid with a binder and spray drying [1]. If necessary, the agglomerates are partially sintered to improve structural integrity.

The next problem is forming a nanostructured coating on the substrate. This is not trivial, since the agglomerates are greatly heated (promoting rapid grain growth) and are at least partially melted. There are three mechanisms for creating or retaining a nanoscale microstructure: avoiding melting or grain growth of the feedstock (very difficult), inclusion of nanoscale particles with very high melting temperature that remain solid while the rest of the material melts, or formation of a nanostructure during solidification of the sprayed material upon impact. The last mechanism occurs in composites consisting of two or more immiscible phases (as is the case for Al₂O₃ and TiO₂) and results from solid state decomposition of a single, metastable phase formed by rapid solidification during impact.

Therefore, while plasma spraying of nanostructured materials is known, there are significant limitations on the process, available compositions, and resulting coated product.

The obvious parameter by which to judge a “wear resistant coating” is wear rate. Wear can be termed as either sliding or abrasive. Both are measured by running a “wearing” medium over the surface and measuring weight loss. For many coatings, and particularly for brittle materials such as ceramics, this parameter can be misleading. The wear resistance of coatings in use today is outstanding, with wear rates orders of magnitude less than the uncoated surface.

Brittle coatings, however, usually do not fail by “wearing out”, but rather suffer from cracking, delamination and spallation. At least as important with respect to the usefulness of a coating on a product as wear resistance are bond strength (adhesion to the substrate) and toughness (the ability to withstand an impact or applied strain). It is in these additional properties that the nanoceramic coatings excel to a remarkable degree.

The bond strength of the nanostructured coatings (e.g., tensile pull strength), is about double that of a conventional coating, S. Sengupta and A. Kumar, “Nano-Ceramic Coatings—A Means of Enhancing Bit Life and Reducing Drill String Trips Normal access”, IPTC 2013: International Petroleum Technology Conference, Asset Integrity I (Mar. 26, 2013), earthdoc.eage.org/publication/publicationdetails/?publication=69795, expressly incorporated herein by reference. The toughness of the nanostructured Al₂O₃—TiO₂ coatings is extraordinary. Conventional (non-nanostructured) ceramic coatings show cracking and spalling. The nanostructured coating deforms along with the substrate and no macroscopic cracking is observed. A blow from a hammer severe enough to deform a steel substrate would not be sufficient to cause failure in the coating. This toughness translates into greater wear resistance, which is two to four times greater than that of the conventional coating. L. Kabacoff, “Nanoceramic Coatings Exhibit Much Higher Toughness and wear Resistance than Conventional Coatings”, AMPTIAC Quarterly V. 6, No. 1, U.S. DoD DTIC Spring 2002, pp. 37-42, ammtiac.alionscience.com/pdf/AMPQ6_1ART05.pdf, expressly incorporated herein by reference.

SUMMARY OF THE INVENTION

Plastic products are now rapidly replacing a myriad of cookware items traditionally used by glasses and ceramics due to their durability, safety, and low manufacturing cost. Despite this trend, some people still prefer using expensive and more fragile ceramic/glass ware because the plastics can deteriorate over time after exposure to foods, which generate malodor, bad appearance, or color change.

Nano-Ceramic Coatings can be used to prevent these drawbacks while still retaining the advantages of the plastic products, as the coating only alters the surface of the plastics and not their bulk properties. The surface coating, however, adds functionality to the plastics, such as a self-cleaning property and disinfectant capabilities that result from a photocatalytic effect of certain ceramic systems. These ceramic coatings can also provide non-stick surface and higher temperature capabilities for the base plastics without using ceramic or glass materials.

Titanium oxide (TiO₂) and zinc oxide (ZnO) are good candidates for a nano-ceramic coating to deposit on plastics or plastic films used in the cookware and kitchenware. Both are a wide band gap semiconductor (3.0-3.2 eV for TiO₂ and 3.2-3.3 eV for ZnO), so they exhibit a photocatalytic property under UV light. This will lead to decomposition of organic compounds proximate to the coating, on exposure to sunlight or fluorescent lighting.

Water can be decomposed, using UV light, into oxygen and hydrogen, without the application of an external voltage, according to the following scheme:

TiO₂ or ZnO+hv→e⁻+h⁺

2 h⁺+2 H₂O→2 H⁺+2.OH→2 H⁺+H₂O₂

e⁻+O₂→.O₂ ⁻

2.O₂ ⁻+4 HO.+H⁺→2 H₂O₂+2 O₂+H₂

H₂O₂→2 HO.

Ultimately, the hydroxyl radicals (.OH) are generated in both the reactions. These hydroxyl radicals are very oxidative in nature and nonselective with redox potential of (E₀=+3.06 V). The hydroxyl radicals react with organic compounds to oxidize them, and often produce decomposition products. Oxidized and decomposed products tend to become more hydrophilic, and therefore can be more easily washed off by water, so the need for detergents may be reduced. See, Biplab Kumar Roy, Guangneng Zhang, and Junghyun Cho, “Titanium Oxide Nanoparticles Precipitated from Low-Temperature, Aqueous Solutions: III. Thin Film Properties”, J. Am. Ceram. Soc., 95 [2] 676-683 (2012). The hydrogen peroxide is toxic to microorganisms. A highly crystalline film with a large surface area for the reaction is important for good photocatalytic (photovoltaic) performance of these oxides.

Low-temperature processing (<100° C.) is important to generating these ceramic coatings on plastic (polymeric) substrates, and especially thermoplastics, without destroying or modifying the underlying substrate.

One way of processing nano-ceramic coatings at low temperatures (<90-100° C.) is to take advantage of in-situ precipitated nanoparticles and nanostructures grown from aqueous solution. Solution based deposition techniques (thermohydrolytic or electrochemical) can generate oxide thin films at very low temperature and low cost. Such solution deposition technique relies upon hydrolysis for converting soluble metal salts into precipitates of metal oxides, and under controlled conditions, the precipitates are nanostructured. These nanostructures can tailor ceramic film formation and the subsequent microstructure development. In addition, aqueous solution deposition provides “environment-friendly” processing without toxic or flammable solvents. Low temperature processing has also shown versatility to generate various nanostructures. The growth of low-dimensional nanostructures (0-D, 1-D, 2-D) provides a means of enhancing the crystallinity of the solution-prepared films that is of importance for photocatalytic performance.

The present technology can generate durable, fully functional, and nano-ceramic coatings (TiO₂, ZnO) on plastic materials (silicone, Teflon, PET, PEN, acrylics, polyethylene, polypropylene, polycarbonate, PEEK, etc.) that possess both photocatalytic oxide properties and flexible plastic properties. Processing cost can be low, and does not require expensive equipment. Further, the process is scalable to permit implementation on a large scale. TiO₂ and ZnO are generally non-toxic, and therefore may be used in food environments.

According to one embodiment, ZnO film deposition is preferred due to the strong crystalline nature of the film deposited, as compared to that of TiO₂ films under similar processing conditions. The forced-hydrolytic deposition of zinc oxide from different soluble salts (e.g., zinc acetate, zinc nitrate and zinc chloride) all produced highly crystalline structure. With a seed layer mediated, low-temperature hydrothermal and electrochemical method (<90° C.), vertically aligned ZnO nanorods were grown. These highly crystalline nanorods dramatically increase surface area within the film, thereby enhancing the photovoltaic efficiency of the device.

The present technology therefore provides a low temperature process for laying down a thin layer of nanostructured ceramic onto surfaces, compatible with plastic products, such as kitchenware, including spatulas, bowls, containers, plastic flatware, serving dishes, wrapping films and even products that are used in hot environments such as in the oven or on a grill (providing the substrate, e.g., plastic, is already capable of withstanding those temperatures). The coating further typically has the property of keeping the plastic from absorbing odors or stains, and would not generally interfere with the plastic's other bulk properties, such as flexibility and light weight. It will also help keep the item's shape, which can deform over repeated use. The coating can, if desired, increase the durability of the item being coated and extend its useful life. It can also be provided to add stiffness to an item, if desired, and some ceramic feel, when held.

The type of substrate is non-critical, and in particular an aspect of the present technology permits coating of temperature-sensitive substrates with a photocatalytic ceramic layer, which may also have advantageous mechanical and chemical properties. The configuration of the substrate is also non-critical, and in particular, the process is not limited to coating of planar surfaces. The surface should be hydrophilic, which is typically achieved by having a preponderance of hydroxyl moieties on the surface. In some cases, a substrate is formed of a clean hydrophilic material, and no modification is required. In other cases, the surface may need to be cleaned, and a cleaning solution such as pirhana (H₂O₂/H₂SO₄) or base pirhana (H₂O₂/NH₄OH) is often suitable, since this cleans and hydroxylates. In some cases, a strong acid or base is sufficient to clean an otherwise hydrophilic surface. Often, an oxygen plasma treatment after cleaning is also useful to ensure the hydrophilicity of the surface.

The substrate surface may be dense or porous. A smooth non-porous surface is useful to form traditional ceramic film coatings, though in some cases a seed layer is provided to help form a high quality film with controlled crystallinity and tailored microstructures. Such films may provide both photocatalytic properties and mechanical/chemical properties.

A porous or otherwise high surface area surface, such as wood, natural fibers, foams, woven or non-woven fabrics may also be coated or in some cases, impregnated with the nano-structured ceramic particles or nanorods. In this case, it is typically the photocatalytic properties which are predominant.

The coating process typically proceeds either with the formation of nanoparticles in solution near the surface of the substrate to be coated, with an agglomeration of particles and densification of the particles at the surface, or with the nucleation and growth of nanostructures at the surface of the substrate to be coated. This process is typically driven by a supersaturation of a solution. Three methods are available. First, immersion in a supersaturated solution leads to surface deposition. Second, a solution may be sprayed onto a surface, in either a fully controlled environment (to control, e.g., temperature, headspace gas composition and thus solution pH, etc.), or in air, with deposition occurring by precipitation of nanoparticles in the droplets, and the mechanical force of the spray. Third, an electrochemical reaction may be provided to locally increase the precipitation conditions for ceramic particles at or near the surface to be coated.

Each of these embodiments, according to the present technology, can be conducted with maximum process temperatures below 100-130° C., and in particular, can generally be conducted at 90° C. or below. In some cases, temperatures at 60° C. or below are employed. Further, some embodiments employ mild reagents that permit use of reactive or fragile substrates.

A range of natural and synthetic polymers, and blends/composites may be employed. Natural materials that may be coated include wood (and wood composite materials), paper, cardboard, bamboo, fibers such as cotton, linen, wool, silk, leather, hemp, and jute, and polymers such as rubber, gutta-percha, and shellac. Many of these materials are microporous, and inherently hydrophilic, though various treatments during manufacture may render then less hydrophilic or hydrophobic, and therefore an initial cleaning and treatment may be required to increase hydroxylation of the surface. A coating on these types of materials will tend to be integrated in the surface region, and only after the pores and crevices are filled, will a more continuous coating form. The filling of the pores and crevices will tend to alter the mechanical properties and feel of the material. In some cases, the photocatalytic coating will tend to degrade the substrate material over time; however, lignin-based materials may have an ability to persist under oxidizing conditions for some time.

Synthetic fiber materials, which may be woven or non-woven, include polyester, acetate, acrylic (acrylonitrile), viscose, cellulose acetate, olefin, aramids (e.g., Kevlar), polybenzimidazole, orlon, vectran, polylactic acid, nylon, lastex (latex), rayon, spandex, viscose, polypropylene, fiberglass, carbon, polyvinyl chloride, polytetrafluoroethylene (PTFE), polyethylene (ultra-high molecular weight, high molecular weight, high density, medium density, low density, ultra-low density), urea-formaldehyde, and various reconstituted cellulose fibers. In general, these tend to have lower fiber porosity than natural fibers, and generally require an initial treatment to increase hydrophilicity. Note that, in some cases, the fiber may be manufactured with a hydrophilic copolymer or block copolymer that provides inherent hydrophilicity. Otherwise, a post treatment, such as piranha cleaning and oxygen plasma treatment, may be used to establish a hydrophilic surface. As with natural materials and fabrics, the synthetic fibers do not provide a flat surface or expanse for deposition of a coating, and therefore the precipitated nanoparticles or nanorods will tend to fill porosity and crevices before forming a more continuous coating outside the material.

In the case of high quality, elongated zinc oxide nanorods, the deposition on a fabric or surface with a conductive base can result in a piezoelectric generator, which produces a current based on movement. See, e.g., Azam Khan, Mazhar Ali Abbasi, Mushtaque Hussain, Zafar Hussain Ibupoto, Jonas Wissting, Omer Nur and Magnus Willander, “Piezoelectric nanogenerator based on zinc oxide nanorods grown on textile cotton fabric”; Naveed Sheikh, Nitin Afzulpurkar, and Muhammad Waseem Ashraf, “Robust Nanogenerator Based on Vertically Aligned ZnO Nanorods Using Copper Substrate”, Appl. Phys. Lett. 101, 193506 (2012); dx.doi.org/10.1063/1.4766921; Journal of Nanomaterials, Volume 2013 (2013), Article ID 861017, dx.doi.org/10.1155/2013/861017; Zhong Lin Wang and Jinhui Song, “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science v. 312 pp. 242-246 (2006), each of which is expressly incorporated herein by reference.

More generally, the substrate material may be Polyester (PES); Polyethylene terephthalate (PET); Polyethylene (PE); High-density polyethylene (HDPE); Polyvinyl chloride (PVC); Polyvinylidene chloride (PVDC); Polyvinylidene fluoride (PVDF) Low-density polyethylene (LDPE); Polypropylene (PP); Polystyrene (PS); High impact polystyrene (HIPS); Polyamides (PA) (Nylons); Acrylonitrile butadiene styrene (ABS); Polyethylene/Acrylonitrile Butadiene Styrene (PE/ABS); Polycarbonate (PC); Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS); Polyurethanes (PU); Maleimide/Bismaleimide; Melamine formaldehyde (MF); Plastarch material; Phenolics (PF); Polyepoxide (Epoxy); Polyetheretherketone (PEEK); Polyetherimide (PEI); Polyimide; Polylactic acid (PLA); Polymethyl methacrylate (PMMA); Polytetrafluoroethylene (PTFE); Urea-formaldehyde (UF); Furan; Silicone; and Polysulfone.

According to some embodiments, the coated product is disposable (i.e., made for one time or seasonal use), and in such form may comprise a biodegradable or environmentally degradable polymer. According to one embodiment, the photocatalytic coating is capable of rapidly degrading the substrate, such that after the single or limited use, the substrate rapidly degrades under ultraviolet (UV) illumination or natural sunlight. According to other embodiments, the coated product is designed to be durable, and may be fabricated using a substrate material such as glass, ceramic, wood or metal. Typically, short duration use plasticware is made from different (and less costly) materials than long term use plasticware; further, products intended to be disposable tend to be photodegradable or biodegradable, while durable products typically avoid spontaneous degradation materials. The coating may be deposited directly on the plastic substrate, or deposited on an intermediate layer, such as a metallic or conductive film. See, U.S. Pat. Nos. 8,621,755, 8,176,641, 6,983,542, 5,280,052, 5,177,124, each of which is expressly incorporated herein by reference.

The present technology provides various benefits, resulting from the new form factors enabled. For example, photocatalytic drapes, curtains or blinds permit deodorizing a room using sunlight as a source. Medical devices, such as intravenous lines, catheters, and other transcutaneous devices may be coated to provide antibacterial properties based on the ultraviolet light emitted by fluorescent bulbs in a medical environment. More generally, all surfaces in a medical environment are subject to coating, including beds, headboards, siderails, etc., bed stands, medical equipment, trays, cups, pitchers, knives, forks, spoons, bedpans, trash receptacles, medical and surgical device packaging, and a full range of materials and configurations. The deposition process does not include any toxic products, and often does not unintentionally impair material properties.

Typically, the ceramic coating is a final or near-final manufacturing step, because the coating can be disrupted or cracked by various forces. As a result, in many cases, even relatively heat resistant substrates, which typically can withstand 100° C. as a bulk material, suffer deformation or degradation when in the form of a finished product.

Another product enabled by the present technology is photocatalytic water disinfection and treatment systems in third-world environments. For example, the coating may be applied to various clean, hydrophilic surfaces, and when properly applied, will produce hydroxyl radicals and hydrogen peroxide upon exposure to sunlight and moisture. Therefore, water troughs exposed to sunlight can disinfect the contents, and degrade dissolved organic compounds. The troughs can be coated in situ or near the place of use, and because the choice of materials is not narrowly constricted, a photocatalytic coating may be provided for existing installations, and in new installations, without displacing incumbent systems, suppliers, or maintenance systems. For example, a spray-applied coating driven by a steam generator to provide pressure and heat (to, e.g., 75-90° C. at the point of application) in a modestly controlled environment, using the zinc acetate technology discussed below, is used to coat plastic tubes or troughs through which contaminated water flows during daylight. (During night time or inclement weather as required to meet demand, UV light may be provided synthetically, or other source of disinfecting agent supplied).

The hydrogen peroxide and hydroxyl radical are somewhat toxic to organisms living in the water, and therefore the technology can also be exploited to reduce mosquito populations. In some areas of the globe where malaria and other insect borne diseases are endemic, standing water in old tires has been identified as a significant breeding ground for mosquitos. Therefore, if the inner wall of a tire is coated during manufacture with a photocatalytic coating, after it is removed from a vehicle, the coating should remain, and interact with sunlight to render the tire an inhospitable environment for mosquito larvae. Other configuration traps may be provided to lure female mosquitos into laying their eggs in a self-sterilizing environment.

A biocidal device is therefore provided, comprising at least one surface configured to retain natural rainwater and be exposed to sunlight, the at least one surface being coated with a nanostructured ceramic coating having a thickness in excess of 100 nm, formed by a deposition of at least one of titanium dioxide and zinc oxide nanostructures from a supersaturated aqueous ceramic precursor solution in a deposition process which does not require the at least one surface to be heated above 100° C., wherein the biocidal device produces larvicidal reaction products of rainwater when exposed to the sunlight.

Another environment for application of the technology is bathrooms. For example, toilet seats, counters, cabinets, floors, walls, tiles, sinks, faucets, hardware, etc., may be provided with a photocatalytic coating. Advantageously, when the regular light is turned off, a UV light source (gas discharge, fluorescent, LED, etc.) may be illuminated to drive the photocatalytic process. Personal articles, such as toothbrushes, combs, hair brushes, etc., can also be coated. In some cases, in the case of personal articles, a cabinet or case may be provided which provides a source of UV light when the object is not in use. Similarly, a dish washer may include an internal UV light source, to activate the photocatalytic effect for coated items contained inside. Indeed, the entire inside of the dish washer (or other enclosed space) may be coated with the photocatalytic coating, since this will generally increase the levels of hydroxyl radicals in the washwater, which in turn will facilitate cleaning of the contents.

The technology may also be used to coat portions of a car or other vehicle. The coating is provided, for example, on the exterior or interior surface(s), and the photocatalytic property is exploited to facilitate self-cleaning.

Similarly, in a clothes washer, the various surfaces, such as the drum (outer surface) and inner wall of the wash chamber provide high surface areas that may be coated with a photocatalytic material, and in use may be exposed to ultraviolet light. These surfaces are often metal or enamel coated, and not intrinsically heat sensitive; however, as the washing machine is fabricated, various heart sensitive elements are added. Therefore, while these surfaces are not plastic, they may benefit from the present technology. In use, the wash water is enriched in hydroxyl radicals as a result of the photocatalysis, resulting in reduced need for detergent and bleach, and reduced odor and possible bacterial or fungal contamination.

In other embodiments, the articles to be coated are in the kitchen or dining environment. Some of these coated plastic products according to the present technology are designed to react with organic compositions from food resulting in stains, odor absorption, or discoloration of the plasticware. Therefore, under exposure to ultraviolet light or sunlight, the organic compositions are oxidized and degraded, which can directly bleach many stains, and otherwise solubilize organic debris. In some environments, UV light is not naturally present. Therefore, according to one embodiment, a UV light source is provided within a cleaning environment, such as a dish washer, conveyor washer, plastic bus tub, dish rack, table or counter. The UV light source may be, for example, a gas discharge lamp or light emitting diode. In a kitchen, the areas that may be coated include utensils, plumbing fixtures, counters, walls, floors, tables, appliances, etc. For example, in a refrigerator or freezer, an ultraviolet lamp may illuminate photocatalytic coated surfaces, resulting in odor reduction, antibacterial effect (both direct from UV exposure, and indirect from hydrogen peroxide and hydroxyl radicals). In some cases, for example the refrigerator, a control system is provided to induce the photocatalytic effect when needed, and save energy and avoid possible side effects of UV light when not required. Assuming odor control is a goal, an odor sensor, such as a semiconductor sensor (possible in the form of a thermistor) or voltammetric sensor, is provided. Other odor sensors include MEMS/nanocatilever sensors, IS-FETs, and enzymatic sensors, and the like. In any case, the UV lamp, and in some cases, a source of moisture, are activated when odors are detected. The system can be a closed loop control system, or triggered periodically for a fixed cycle, i.e., after triggered, the process follows a predetermined course, such as 1 hour of UV and moisture, regardless of the trigger level and the measured effect of the process.

A photocatalytic water treatment system is therefore provided, comprising at least one surface configured to be wet with water and to be exposed to ultraviolet light, the surface being coated with a nanostructured ceramic coating having a thickness in excess of 100 nm, formed by a deposition of at least one of titanium dioxide and zinc oxide nanostructures from a supersaturated aqueous ceramic precursor solution in a deposition process which does not require the at least one surface to be heated above 100° C.; and an illumination system configured to illuminate the surface with ultraviolet light of sufficient intensity to treat water in the water flow path.

The surface may be an exposed wetted surface of a clothes washer isolated from contact with clothes, the illumination system further comprising a source of UV light configured to supply UV light during operation of the clothes washer to the exposed wetted surface.

The surface may also be an interior surface of a refrigerator, the illumination system further comprising a source of UV light configured to supply UV light during operation of the refrigerator to the interior surface and a source of moisture to wet the interior surface. Advantageously an odor detection sensor may be provided, along with an automated control to control at least the UV light in dependence on an output of the sensor.

With time, normal plasticware can also warp and lose its shape, for example due to residual stresses from a manufacturing process (e.g., injection molding) and the exposure to heat and UV radiation. Further, the use of plastics in microwave ovens or with hot food causes concerns for some consumers who worry about toxic chemicals leaching out of the plastic, even when the plastic is BPA-free (Bisphelol A or BPA is an additive for plastic and is also used in some plastic coatings products that has been found to cause some harm in laboratory animals, though not specifically in humans). The coating may be engineered to maintain the surface condition and configuration of the plasticware, and thus avoid or reduce deformation over time, and/or leaching of materials out of the plasticware. Further, the nanostructured ceramic coating generally does not itself contain leachable organic components, and therefore provides a barrier.

The coating also has photocatalytic properties that can reduce dirt buildup (i.e., require less soap or detergent to clean) and make the coated products easy to disinfect under UV light. Plus, having a ceramic coating over plasticware can repel potential bacterial buildup that can degrade the plastic. Finally, while the coating is ceramic, it does will not chip or break off if an item coated with it, if it is dropped.

The coating may be transparent, opaque, or tinted, and preferably includes inorganic components that do not disrupt the photocatalytic effect of UV light on the ceramic nanostructures, and avoids organic components, especially those that would degrade when subject to hydroxyl radicals from photocatalysis, and organic or inorganic components that would defeat the photocatalytic process by, for example, quenching free radicals, filtering UV light, significantly competing for UV photon capture, or provide a secondary path for release of the activation energy of the UV photons absorbed by the ceramic.

In some cases, the coating forms part of a photovoltaic cell generator or piezoelectric generator; these implementations typically compete for the energy needed to drive a photocatalytic process, and therefore within a given region of an object, there various implementations are not provided together. However, different regions of the substrate may have different functions, and a single coating may provide a basis for different end results. Therefore, while a photocatalytic product is one type of preferred embodiment, it is by no means the only useful result of depositing a nanostructured ceramic coating on a substrate.

Other products which may be produced using the technology include hair straighteners, having ceramic-coated plastic heating plates; window coatings (Yanfeng Gao; et. al. “Nanoceramic VO₂ thermochromic smart glass: A review on progress in solution processing,”; Volume 1, Issue 2, March 2012, Nano Energy, www.sciencedirect.com/science/article/pii/S2211285511000255 (accessed Sep. 4, 2012), expressly incorporated herein by reference); polyvinyl siding for residential, utility, and commercial buildings; laboratory plasticware; and dishwasher inner surfaces (Carter, John David; et. al. “Rinse aid surface coating compositions for modifying dishware surfaces,” Aug. 8, 2006, U.S. Pat. No. 7,087,662, expressly incorporated herein by reference). Further products which may benefit from the nanostructured ceramic coating include screen protectors and/or oleophobic (anti smudge) lenses or surfaces for smartphones, tablets and touchscreens, smartphone cases, keyboards, automated teller machine (ATM), and other electronic device user interfaces, as well as stainless steel appliances and other exposed surfaces on which fingerprints may be evident. Dyes can be included in the coating as a finish to give appliances any tint, while enjoying the wear resistant and anti-smudge advantages of the coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

General procedures of precursor solution preparation and titania film deposition are discussed in G. Zhang, B. K. Roy, L. F. Allard, and J. Cho, “Titanium Oxide Nanoparticles Precipitated from Low-Temperature Aqueous Solutions: II. Thin-Film Formation and Microstructure Developments,” J. Am. Ceram. Soc., 93 [7] 1909-15 (2010), expressly incorporated herein by reference.

A preferred method for coating a plastic item is as follows. The surface of the plastic, which may have a mold release composition or other residual coating on it, is first cleaned, for example with freshly prepared piranha cleaning solution, i.e., H₂O₂ and sulfuric acid. A typical mixture is 3:1 concentrated sulfuric acid to 30% hydrogen peroxide solution, though a range of 2:1 to 7:1 may be used. Cleaning is conducted for 1-10 minutes at an appropriate temperature below 100° C., e.g., 60-90° C., though care is exercised to avoid significantly degrading the substrate, and the cleaning is ceased as soon as the surface is uniformly wetted and clean. The substrate is then dried in dry nitrogen gas (N₂) blow, and treated with an O₂ plasma (Harrick Plasma, Ithaca, N.Y.) for 15 min to render the surface hydrophilic.

A desired amount of hydrochloric acid (HCl, 36.5%-38%, J. T. Baker, Phillipsburg, N.J.) was first dissolved in ice-cold DI water (Barnsted E-pure, resistivity 18-20 MΩ-cm) followed by slow injection of adequate amount of titanium chloride (TiCl₄, 99.99%, Alfa-Aesar, Ward Hill, Mass.) in a parafilm-covered glass bottle for supersaturation. The method for calculating supersaturation at the deposition temperature is addressed in G. Zhang, B. K. Roy, L. F. Allard, and J. Cho, “Titanium Oxide Nanoparticles Precipitated from Low-Temperature Aqueous Solutions: I. Nucleation, Growth, and Aggregation,” J. Am. Ceram. Soc., 91 [2] 3875-82 (2008), expressly incorporated herein by reference. Once prepared, the solutions were kept refrigerated until utilized.

The substrates may be seeded, by coating with compatible nanocrystals, such as by spin coating (e.g., for flat surfaces) or dipping in a seeding solution.

The substrates are then placed in a beaker containing freshly prepared precursor solution. The beaker was placed in an oil-bath, preset at 60-90° C., to perform the deposition. The pH of the solution is maintained by addition of a suitable acid, such as HCl for TiCl₄ (pH <1.5).

The process is preferably conducted at temperatures below 90° C., both to avoid damage to the substrate, and because the low temperature maintains a slow reaction rate and higher quality smaller crystals. As temperature increases, the reaction rate increases, and larger crystals with higher crystallinity result.

The coating may be facilitated by an electrochemical process. The substrate is selected as one which is inherently conductive, or coated with a conductive surface, such as a metal. In this case, hydrogen peroxide is added to the precursor solution, for example, 10 mM hydrogen peroxide in 5 mM TiCl₄ in 3:1 methanol-DI water. For example, the substrate is held at a cathodic deposition potential, with current held at a level which does not result in apparent hydrogen generation (bubbling), which might reduce coating quality. For example, the cathode voltage is held between about −3V to −5V with respect to a platinum foil reference electrode (anode) in the solution. pH and voltage may be adjusted to control both hydrogen bubble formation and corrosion of the conductive substrate. See, Biplab K. Roy, Guangneng Zhang, Roy Magnuson, Mark Poliks, and Junghyun Cho, “Electrodeposition of Titania Thin Films on Metallic Surface for High-k Dielectric Applications”, J. Am. Ceram. Soc., 93 [3] 774-781 (2010), expressly incorporated herein by reference.

The deposition is conducted to produce a coating of the desired thickness, and may be monitored by pH change (and amount of acid needed to titrate the solution to maintain pH), time and electrical current, etc., or by mechanical or functional measurements. The precursor solution is changed at every hour to increase the deposition rate and to avoid any heavy particle agglomeration.

The surface morphology and crystallinity of the resulting thin films can be controlled by changing solution parameters. A thermodynamic parameter, supersaturation (S), has been identified as a key controlling factor to tailor such variations.

After desired deposition periods, films were cleaned with ethanol and dried under mild N₂ blow.

Titania particles form electrochemical conversion of TiCl₄ to TiO₂ in the solution, via thermal-energy-driven homogeneous nucleation. The thin film formation occurs by attraction and assembly of nanoparticles on substrate surface. Precursor solution environment not only determines the nanoparticle assembly and the film microstructure, but also influences the phase of titania (amorphous, anatase, or rutile).

Films obtained from very low S (˜3.9) solution typically have a distinct “leaf-like”-structured surface morphology, with traces of anisotropic structured growth extending from the substrate surface to the top edge of the film. Growth rates in low S precursor are higher than in high S precursor. With increasing supersaturation (i.e., S ˜232.8), bulk precipitation becomes more dominant with less contribution toward film formation and hence, the film growth rate decreases. A low S solution contains more HCl and less TiCl₄. Increased HCl imparts a common ion (Cl—) effect and prevents dissociation of TiCl₄, and low supersaturation can therefore be achieved. In this process, high S solution inherently has higher pH, whereas low S solution shows lower pH. Three phases of titania (namely rutile, anatase, and amorphous) are formed.

At very high supersaturation, the rate of hydrolysis is much faster than condensation. This situation can lead to random polycondensation of hydrolyzed octahedra and generation of amorphous phase. Therefore, at higher supersaturations, anatase phase surrounded primarily with amorphous titania is obtained. Amorphous phase content increases with increase in supersaturation and leads to the formation of denser and smoother film morphology. In contrast, low S conditions primarily produced directed rutile-type crystalline growth and porous films with rough topology.

Although anatase and rutile crystals can be observed in the deposited films, the presence of amorphous phase cannot be ignored in overall film morphology. Due to rapid hydrolysis characteristics of Ti⁴⁺, even in very controlled deposition conditions, polycrystalline films contain an amorphous phase along with nanocrystalline particles. Therefore, in all situations, it is important to realize the composite nature of the films with some dominating phases. From low S precursors, rutile phase appears as a dominant one. As the supersaturation increases, the anatase phase first dominates in the film structure and the amorphous phase becomes prevalent at even higher supersaturations. In low S deposited rutile-containing films, porosity and segmented structure of aligned plates limits is evident. The optical absorption spectrum of rutile films obtained from low S solution is markedly different from that of films obtained from higher supersaturations.

Highly acidic low-supersaturation solutions produce a rutile-type of crystallinity and porous morphology, whereas the higher supersaturation results in the formation of amorphous and anatase phase with a denser microstructure. UV-Vis studies reveal a distinct difference in the optical absorptions between films formed from low S and high S TiCl₄ precursor solutions. The rutile-based films displayed a lower optical band gap than the films containing anatase/amorphous phases. Due to their densely packed particulate structure, the films obtained from high S showed higher mechanical properties than the porous rutile films. Dielectric properties of the rutile films were, however, superior to the anatase/amorphous films because of significant difference in dielectric constants among amorphous, anatase, and rutile phases. This offers a way of tuning thin film dielectric properties by manipulating the phase evolution with controlled solution parameters. The photoelectrochemical response is higher for the rutile-containing films, attributable to higher porosity (leading to more dye absorption, higher interaction area), higher refractive index, better crystallinity, and larger thickness of the low S generated films compared to their high S counterparts.

EXAMPLE 2

Zinc oxide (ZnO) films consisting of vertically aligned nanorods may be hydrothermally grown on a seed layer at e.g., 90° C. using two alternate precursors (zinc acetate, zinc nitrate). Vertically grown nanorods exhibit the (002) out-of-plane texture and their size, alignment, density, and growth rate can be controlled by both solution and seed layer conditions. A continuous or stepwise deposition may be implemented. A seed layer, e.g., ZnAc₂ may be deposited and cured at temperatures as low as 100° C. In-situ precipitated nanoparticles and nanostructures from aqueous solution are provided. See, Sunghee Lee, Biplab Kumar Roy, and Junghyun Cho, “Vertically Aligned ZnO Nanorods Grown by Low-Temperature Solution Processing”, Japanese Journal of Applied Physics 52 (2013) 05DA09, expressly incorporated herein by reference.

Zinc oxide (ZnO) is a direct wide band gap (3.4 eV) semiconductor, which is comparable to TiO₂, while having several advantages over TiO₂ such as easy crystallization at low temperature, 1D anisotropic growth, and high electron mobility. A hydrothermal process to produce the film employs low process temperatures, which permit use of flexible polymer substrates. Compared to other solution-based techniques which utilize open bath, the hydrothermal processing provides high controllability of nanostructures because of the mild deposition condition resulting from the higher solubility of zinc ions than, for example, titanium ions. Vertically aligned nanorods or nanotubes have shown some advantages over the nanoparticle clustered structures for enhanced photovoltaic (PV) properties due to their faster electron transport and reduced charge recombination. A seed layer may be provided to assist in in aligning the nanorod structure on the substrate.

The surface of the plastic substrate, which may have a mold release composition or other residual coating on it, is first cleaned, for example with freshly prepared piranha cleaning solution, i.e., H₂O₂ and sulfuric acid. A typical mixture is 3:1 concentrated sulfuric acid to 30% hydrogen peroxide solution, though a range of 2:1 to 7:1 may be used. Cleaning is conducted for 1-10 minutes at appropriate temperature, though care is exercised to avoid significantly degrading the substrate, and the cleaning is ceased after the surface is uniformly wetted and clean. The substrate is then dried in dry nitrogen gas (N₂) blow, and treated with an O₂ plasma (Harrick Plasma, Ithaca, N.Y.) for 15 min to render the surface hydrophilic.

The treated substrate can be coated with a thin layer of a 1:1 molar ratio of zinc acetate dihydrate and ethanolamine in 2-methoxyethanol (all three from Alfa Aesar), in a concentration range of e.g., 50 mM-750 mM, though other concentrations may be employed as appropriate. The solutions may be pre-heated at 60° C. for 40 min in a water bath before coating, and cured at less than 90-100° C.

On the seed layer, ZnO films may be grown by hydrothermal deposition. Two types of precursors may, for example, be used: i) 20 mM zinc acetate dihydrate and 20 mM hexamethylenetetramine (HMT; Alfa Aesar); ii) 25 mM zinc nitrate hexahydrate (Alfa Aesar) and 25 mM HMT aqueous solution. Seed layer coated substrate is immersed into the precursor solution. The deposition may be conducted at 60-90° C. The deposition may be, for example, 2-8 hours, and may be repeated to build up layer thickness and density. For example, 4 2-hour sessions may be conducted with a gentle wash and solution replacement between each deposition. During the hydrothermal deposition, bulk precipitates may form, in the precursor solution, and therefore the solution may be replaced with a freshly prepared solution every 1-2 h.

A hydrothermal spray coating process is also possible, in which particles are formed in a hot supersaturated solution and sprayed with force on an object, to provide a mechanical impact effect to facilitate agglomeration of particles at the surface of the substrate. The solution can be allowed to dry after spraying. In a spray coating embodiment, it is useful to maintain the substrate at elevated temperature, e.g., 60-90° C.

After the hydrothermal deposition, the ZnO films were rinsed with deionized water, and blow dried with nitrogen gas.

Synthetic oxide films in aqueous solution are formed under an accelerated hydrolysis environment for a relatively short period. Such hydrolysis process of precursor species strongly depends solution parameters such as pH, concentration and temperature. The solubility of the oxides and their hydroxides need not be known, and the thermodynamics data may be used to calculate equilibrium solubility for the stable phases, from which the degree of supersaturation S can be calculated. It provides the driving force for nucleation and growth of the oxide nanostructures. G. Zhang, B. K. Roy, L. F. Allard, and J. Cho: J. Am. Ceram. Soc. 91 (2008) 3875.

Depending on the availability of OH— (i.e., with pH of solution) the extent of hydrolysis may vary. In the Zn—OH system, soluble species of Zn (II) ions include Zn²⁺, Zn(OH)⁺, Zn(OH)₂, Zn(OH)³⁻, and Zn(OH)₄ ²⁻. A preliminary calculation indicated that S at pH 7 or lower is extremely small compared to that of Ti—OH, and therefore a complexing agent such as HMT (C₆H₁₂N₄) or dimethylamine borane [DMAB, BH₃NH(CH₃)_(2]) is provided to assist in precipitating a ZnO phase. Due to the complexing agent, Zn²⁺ cation also forms amine complexes such as Zn(NH₃)₄ ²⁺ with NH₃(aq) in moderately basic solution.

A higher degree of supersaturation S can be attained either by increasing temperature or by increasing pH of the solution, so subsequent precipitation can be accelerated.

The nanorods from a zinc acetate precursor solution tend to show straighter and more densely packed structure while those from a zinc nitrate precursor are less vertically aligned and less dense. The morphological difference between the films produced by different precursors is likely due to different pH values of the solutions. The initial pH values for the zinc acetate based precursor and the zinc nitrate based precursor are, for example 6.95 and 6.82, respectively. The difference in pH over 0.1 can in fact make a significant change in terms of the degree of supersaturation, which is the driving force for nucleation and growth of the ZnO nanorod. Therefore, high pH in the case of zinc acetate precursor will yield more nucleation density for ZnO rods and make them more packed and straight during the growth. The effects of ionic species generated from different precursors alters the stabilization of the rod surfaces (particularly, basal plane vs non-basal planes); by inactivating non-basal planes (m-planes) through ion attachment, the aspect ratio can increase and the rod growth can be faster.

Nanorod films may have a thickness range, for example, from 350 to 1700 nm, without cracks or film delamination.

EXAMPLE 3

A polymeric substrate in the form of a molded, extruded, or formed useful article, subject to degradation by extended temperatures in excess of 100° C. is provided. The substrate is initially prepared to ensure a hydrophilic surface. For example, the article may be immersed or coated with piranha cleaning solution (H₂O₂ and sulfuric acid), for a sufficient time to fully clean the surface, but the process is limited to avoid substantial damage to the article. The substrate is then dried and may be treated with oxygen plasma to render the surface hydrophilic. In some cases, the surface may be masked, either to selectively produce hydrophilic properties, or to subsequently block the surface, to produce a latent pattern.

The substrate is, for example, formed from polyethylene terephthalate (PET), PEEK, polyurethane, nylon, epoxides, polyamides, polyaramides, polyvinyl chloride, polystyrene, ABS (acrylonitrile and styrene, toughened with polybutadiene), polyethylene, polypropylene, polycarbonate, Teflon® or other fluoropolymer, silicone, silicone heteropolymer or copolymer, etc. Rubbers and elastomers may also be treated. Films and panes, especially optically transmissive structures, may be employed as well.

The useful article is, for example, a kitchen utensil, an eating utensil (knife, fork, spoon), kitchenware (plate, bowl, cup), tray, table, headboard, cutting board, spatula, container, plastic flatware, serving dish, toothbrush, hair brush, or the like. The useful article can also be a disposable medical device, such as a catheter, intravenous line, suture, or other transcutaneous or patient-contact device, or simply an item provided in the patient room, recovery room, operating or procedure room.

The treated substrate may be pre-seeded per Example 2.

The substrate, which may be pre-seeded, is immersed in a supersaturated solution of ceramic precursor, and the supersaturated solution may be replenished after some period of deposition with an acid having the same counterions as the ceramic precursor cations, to maintain supersaturated deposition conditions. Process temperatures are maintained below a softening temperature of the molded useful article, i.e., below 100° C., and preferably below 90° C. throughout the process.

The deposition proceeds for 2-8 hours with the precursor solution replaced every 2 hours, to form a layer of ZnO nanorods e.g., 1,000 nm thick, and preferably in the range 250-3,000 nm thick.

EXAMPLE 4

A low-density polyethylene mixed with polyisobutene (PIB) or poly[ethylene-vinylacetate] (EVA) copolymer 40-100 gage, biaxially oriented monolayer film is provided. One surface of the film is treated with an oxygen plasma to increase hydrophilicity. The hydrophilic surface is immersed in a supersaturated ceramic precursor solution, to selectively coat the hydrophilic surface with a nanostructured ceramic coating 100-350 nm thick. The resulting product is a ceramic-coated asymmetric cling wrap, with a sticky side and a ceramic coated side. The ceramic coating reduces permeability to oxygen and water, increases handleability, and provides photocatalytic properties. The process conditions are maintained below 100° C. Because of the tight radii that such a film may be subjected to, it is likely that the ceramic coating will suffer cracks if used as a traditional cling wrap. However, portions of the film that are not bent or crushed, should display a high ratio of photocatalytic activity to weight, and may be used to provide a temporary photocatalytic surface.

EXAMPLE 5

A wood product, such as a cutting board, is provided. The wood is treated to ensure hydrophilicity, such as by acid or base, short piranha treatment, enzymatic treatment, or the like, and optionally an oxygen plasma treatment.

The wood is kiln dried at 100° C. and surface of the wood is saturated with 1:1 zinc acetate: and ethanolamine in 2-methoxyethanol, and then dried at 100° C. to leave crystal seeds.

The wood product is then immersed in a supersaturated solution of aqueous 20 mM zinc acetate/20 mM hexamethylenetetramine for two hours or more. The resulting product has a surface which is impregnated and coated with ZnO ceramic nanorods. It is noted that under ultraviolet illumination, with moisture, hydroxyl radicals and hydrogen peroxide are generated, which will tend to degrade the wood, but also degrade odors, food residue, and bacteria. The lignin in the wood is relatively resistant to oxidation, and therefore the reduction in product life is acceptable.

EXAMPLE 6

A woven or non-woven fabric, such as a natural fiber, such as cotton, or linen, or a synthetic fiber such as polyester, nylon, rayon, PET, polyethylene, or the like is provided.

Depending on the fiber type, the substrate is treated to ensure a high degree of hydroxylation, such as by an acid treatment and/or oxygen plasma treatment. Hydrophobic substrates formed of non-porous fibers, such as ultra high molecular weight polyethylene, may be treated with piranha.

The hydrophilic substrate is saturated with 1:1 zinc acetate: and ethanolamine in 2-methoxyethanol, and then dried at 100° C. to leave crystal seeds. The seeded substrate is then immersed in a supersaturated solution of aqueous 20 mM zinc acetate/20 mM hexamethylenetetramine for two to eight hours. The resulting product has a surface which is impregnated with ZnO ceramic nanoparticles.

The fabric may, prior to coating, be formed into a useful article such as drapes or other window treatments. Under ultraviolet illumination, in the presence of moisture, hydroxyl radicals and hydrogen peroxide are generated, which will render the drapes hung in a window or as a room divider in hospital room settings as an air cleaner, to reduce odor tend to reduce bacterial growth and aerosol transfer.

EXAMPLE 7

Metallized plastic silverware or a metallized plastic cell phone case is provided. See, U.S. Pat. Nos. 8,621,755, 8,176,641, 6,983,542, 5,280,052, 5,177,124. The substrate is prepared by treatment with oxygen plasma to render the surface hydrophilic.

A coating is formed by an electrochemical deposition process. The ceramic precursor solution includes 10 mM hydrogen peroxide in 5 mM TiCl₄ in 3:1 methanol-DI water. The substrate is held at a cathodic deposition potential, and maintained at a pH and voltage potential to avoid corrosion of the metallized coating and also avoid hydrogen bubbling, while driving formation of a ceramic coating.

A coating is formed as a single layer or in a series of layers, for example 30 seconds applied potential, 30 seconds altered potential (preferably, a cathodic protection potential for the metalized film) for 4 cycles, to form a ceramic layer of 250-1,000 nm.

It is to be understood that while a certain form of the invention is illustrated, it is not to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and any drawings/figures included herein. The embodiments, methods, procedures and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims.

All patents and publications mentioned in this specification are expressly incorporated herein by reference in their entirety, and may be pertinent to various issues.

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What is claimed is:
 1. A nanostructured ceramic coating having a thickness in excess of 100 nm, comprising titanium or zinc oxide particles precipitated from an aqueous solution of a ceramic precursor on a surface of a substrate.
 2. The nanostructured ceramic coating according to claim 1, comprising titanium oxide.
 3. The nanostructured ceramic coating according to claim 1, comprising zinc oxide.
 4. The nanostructured ceramic coating according to claim 1, configured to be at least one of a photocatalytic coating, a photovoltaic coating, and a piezoelectric coating.
 5. The nanostructured ceramic coating according to claim 1, wherein the substrate comprises an organic polymer.
 6. The nanostructured ceramic coating according to claim 1, wherein the substrate comprises a polymeric substrate, and the surface comprises a conductive metalized coating between the polymeric substrate and the nanostructured titanium or zinc oxide ceramic coating, wherein the nanostructured titanium or zinc oxide ceramic coating is electrochemically deposited.
 7. The nanostructured ceramic coating according to claim 1, deposited in a hydrothermal deposition process.
 8. The nanostructured ceramic coating according to claim 1, wherein the aqueous solution of ceramic precursor is supersaturated.
 9. The nanostructured ceramic coating according to claim 1, comprising a titanium or zinc oxide ceramic crystalline phase within a titanium or zinc oxide ceramic amorphous phase.
 10. The nanostructured ceramic coating according to claim 1, comprising at least one of an anatase phase and a rutile phase surrounded by amorphous titania.
 11. The nanostructured ceramic coating according to claim 1, wherein the aqueous solution of titanium or zinc oxide ceramic precursor comprises at least one of TiCl₄, Zn(Ac)₂, and Zn(NO₃)₂.
 12. The nanostructured ceramic coating according to claim 1, wherein the particles comprise nanorods or nanotubes.
 13. A system, comprising: at least one surface coated with a nanostructured ceramic coating having a thickness in excess of 100 nm, formed by a precipitation of at least one of titanium dioxide and zinc oxide nanostructures from a supersaturated aqueous ceramic precursor solution; and an illumination system configured to supply illumination to the surface.
 14. The system according to claim 13, wherein the surface is photocatalytic, and the surface is configured contact a flow of aqueous solution, wherein the illumination system induces free radicals in the aqueous solution.
 15. The system according to claim 13, wherein the surface comprises an exposed wetted surface of a washer having an aqueous washing fluid, the illumination system further comprising a source of UV light configured to supply UV light during operation of the washer, to cause generation of free radicals in the aqueous washing fluid.
 16. The system according to claim 13, wherein the surface comprises an interior surface of a refrigerator, the illumination system further comprising a source of light configured to supply light during operation of the refrigerator to the interior surface.
 17. The system according to claim 13, further comprising an odor sensor, and a control to control at least the illumination system in dependence on an output of the sensor.
 18. The system according to claim 13, wherein the at least one surface is configured to produce biocidal products of moisture from sunlight.
 19. A catalytic system, comprising a surface coated with a ceramic coating having a thickness in excess of 100 nm, comprising crystalline, nanorod or nanotube ceramic nanostructures, within in an amorphous ceramic phase.
 20. The catalytic system according to claim 19, wherein the ceramic is selected from the group consisting of titanium oxide and zinc oxide, formed by precipitation from an aqueous solution of ceramic precursor. 